In recent years, a certain kind of communication that is used in wireless communications, such as cellular phones and the like, achieves high utilization efficiency of frequency band, and has a high PAPR (Peak to Average Power Ratio) in a wireless signal. In order to amplify a signal having an amplitude modulation component using an AB-grade amplifier which has been conventionally used in an art of wireless communication, it is necessary to be operated in an enough backoff state for maintain linearity
In general, the back off that is same as at least the PAPR is required.
Regarding this, the efficiency of an AB-grade amplifier is best in a saturation state and is decreased as the back off is increased. Due to this, for a high frequency modulation signal having a high PAPR, it is difficult to improve the power efficiency of the power amplifier.
As a power amplifier that amplifies a high frequency modulation signal having a high PAPR at a high-efficiency, there is a polar modulation type of power amplifier. The polar modulation type of power amplifier is used to amplify a high frequency modulation signal that is a wireless communication signal, which includes amplitude modulation and phase-modulated components generated with polar coordinates comprised of amplitude and phase components. The polar modulation type of power amplifier includes an EER (Envelope Elimination and Restoration) system of power amplifier, which can be substituted for the AB-grade amplifier.
FIG. 1 is a block diagram showing a structure of an EER type-power amplifier according to the prior art.
As shown in FIG. 1, signal 110 inputted to an EER type of power amplifier is divided into amplitude signal amplifying route 106 and phase signal amplifying route 103.
In amplitude signal amplifying route 106, envelope detector 105 extracts envelope signal 108 (amplitude modulation component) from input signal 110, which is then amplified in linear amplifier 104. In phase signal amplifying route 103, limiter 102 extracts phase-modulated signal (phase-modulated component) 107 having a constant envelope from input signal 110, which is then amplified in high frequency amplifier 101.
To high frequency amplifier 101 is provided, as a power supply, output signal 109 of amplitude signal amplifying route 106. High frequency amplifier 101 is biased with output signal 109 of amplitude signal amplifying route 106, so that it always operates in a saturation state, thereby outputting modulation signal 111 having synthesized the phase-modulated signal and the envelope signal.
The reason why the EER type of power amplifier, shown in FIG. 1, can improve power efficiency is because it uses a high efficient switching amplifier, such as linear amplifier 104 and because it enables high frequency amplifier 101 always operate in a saturation state.
A typical example of linear amplifier 104 shown in FIG. 1 is shown in FIG. 2.
A signal band treated in amplitude signal amplifying route 106 shown in FIG. 1 is approximately same as a signal band of input signal 110, which is typically several hundreds kHz to several tens MHz. Due to this, linear amplifier 104 can be structured with a D-grade amplifier comprising AD (Analog to Digital) converter 201 that converts envelope signal 108 into a bit stream signal using PDM (Pulse Density Modulation) and the like, switching amplifier 202 and low-pass filter 203, as shown in FIG. 2. Ideally, there occurs no power loss in the linear amplifier.
Furthermore, general high frequency amplifier 101 has a characteristic in which it most efficiently operates in a saturation state. The power efficiency of an EER type-power amplifier is the product of the efficiency of linear amplifier 104 and the efficiency of high frequency amplifier 101.
The EER type of power amplifier shown in FIG. 1 has a tendency in which the efficiency thereof is decreased when the average power of modulation signal 111 is small. Due to this, a variety of attempts have been made to improve the efficiency of the EER type-power amplifier. For example, an EER type of power amplifier of the background disclosed in a Japanese Unexamined Patent Publication No. 2003-304127 is shown in FIG. 3.
A circuit shown in FIG. 3 comprises voltage control device 809 and output power meter 808, which are added to the circuit shown in FIG. 1. A signal inputted from input terminal 801 is inputted to envelope detector 802 and limiter 803. From the signal inputted to envelope detector 802, only an envelope signal is extracted and outputted to linear amplifier 804. Linear amplifier 804 amplifies the inputted envelope signal inputted and outputs the amplified signal to high frequency amplifier 805 as a power supply voltage.
The signal inputted to limiter 803 is converted into a phase-modulated signal having a constant envelope, which is then outputted to high frequency amplifier 805. High frequency amplifier 805 multiplies the envelope signal outputted from linear amplifier 804 by the phase-modulated signal outputted from limiter 803, and outputs the multiplied signal.
The output signal from high frequency amplifier 805 is outputted to output terminal 807 and is supplied to output power meter 808. Output power meter 808 detects an output power of high frequency amplifier 805 and provides voltage control device 809 with information of the detected output power.
Voltage control device 809 controls a power supply voltage that is supplied to linear amplifier 804, based on the information received from output power meter 808. Linear amplifier 804 has a PWM (Pulse Width Modulation) circuit, a switching amplifier and an output filter.
The power amplifier shown in FIG. 1 has the problem in which the efficiency of linear amplifier 104 is lowered when the average power of modulation signal 111 is small, thereby lowering the efficiency of the overall circuit. Meanwhile, in the power amplifier shown in FIG. 3, the power supply of linear amplifier 804 is supplied from voltage control device 809 and the power supply voltage of linear amplifier 804 is changed depending on the output power of high frequency amplifier 805, thereby preventing the efficiency from being lowered.
The EER type of power amplifier shown in FIG. 1 has the problem in which when the average power of modulation signal 111, which is the output signal from the power amplifier, is small, the SNR (Signal to Noise Ratio) thereof is poor, in addition to the above problem in which the efficiency is lowered. This is because the quantization noise of AD converter 201 provided to linear amplifier 104 is constant regardless of the magnitude of envelope signal 108 to be inputted.
In the meantime, the power amplifier shown in FIG. 3 has the problem in which a gain of the overall circuit is varied depending on the output power of high frequency amplifier 805. This is because a gain of the PWM circuit is varied by the power supply voltage. Japanese Patent Laid-Open No. 2003-304127 does not disclose a specific method that solves the above problems. As a result, it is not possible to make the gain of the power amplifier as a desired value and thus it is not possible to make the power of the output signal from the power amplifier as a desired value.